Switch and a switching apparatus for a communication network

ABSTRACT

A switch module for a communication network switch comprises first and second switching units each for communicating with one or more network communication interfaces, and a switch interface connected to each of the first and second switching units and having a pair of communication ports for enabling data to be transferred on to a local communication path which is connected between the local communication ports.

FIELD OF THE INVENTION

[0001] The present invention relates to a switch and switching apparatusfor a communication network, and in particular to a switch and switchingapparatus having redundancy.

BACKGROUND OF THE INVENTION

[0002] A typical communication network switch has a switching core andan array of communication network interfaces connected to the switchingcore. The network interfaces pass data traffic from the communicationnetwork to the switching core which directs the data traffic back to theappropriate communication interfaces for continued transmission over thenetwork. Multiple network interface cards are accommodated in equipmentshelves and are arranged to aggregate network traffic and feed theaggregated traffic to the central switching core. The capacity of aswitching core has a maximum upper limit and therefore a serviceprovider must select a switching core with the required capacity for theparticular network. Since the demand for network bandwidth is notstatic, but typically increases over time, a service provider may eitherchange the switching core from time to time to meet the requiredcapacity, or purchase a switching whose capacity exceeds that of thepresent bandwidth requirements, with a view to connecting additionalnetwork interface cards to the switching core, as the demand for extracapacity increases. One drawback of the first option is that each time aswitching core needs to be upgraded to a higher capacity switching core,network traffic cannot be serviced by the switch and therefore a secondcentral core (providing redundancy to the active core) must be invoked,in which case redundancy is lost, or traffic must be re-routed aroundthe switch, if possible. Customers may also experience a complete lossof service during the period over which the switching core is beingreplaced, and the service provider may suffer a loss of revenue. Adrawback of the second option is that a service provider must invest ina switching core, part of whose potential capacity is not initiallyused, and therefore does not generate revenue, and may not generate anyrevenue for some time. High capacity switches, for example having atraffic handling capacity of the order of terabits are complex andexpensive, and the cost of the initial investment in capacity to meetfuture demand and the period over which the capacity is underused mayrender these switches inaccessible to at least some service providers.

[0003] Communication network switches require a level of faulttolerance, and this is normally provided by duplicating the centralswitching core so that a second core can take over switching functionsof the active core if the active core experiences a fault. The standbycore usually has the same capacity as the active core and therefore theprovision of a second switching core to provide fault tolerancesignificantly increases the cost, particularly for high capacityswitches.

SUMMARY OF THE INVENTION

[0004] According to one aspect of the present invention, there isprovided a switch for a communication network, comprising: a firstswitching unit for communicating with one or more communicationinterfaces, a second switching unit for communicating with the one ormore communication interfaces, and switch interface means forcommunicating with the first and second switching units and having acommunication port for enabling at least one of data to be transferredto the switch interface means and data to be transferred from theinterface means.

[0005] In this arrangement, the switch has first and second switchingunits, each of which is capable of communicating with one or morecommunication interfaces. Accordingly, if one of the switching unitsfails, the other switching unit may be invoked to enable communicationbetween the switch and the communication interfaces to continue. Theswitch further includes a switch interface means associated with theswitching units and which has at least one interface port, and enablesdata to be transferred between the switching units and the interfaceport(s). The interface port is connectable to a local communication pathwhich may be coupled to an interface port of another similar switch,which may communicate with one or more other communication interfaces.Thus, the switch interface enables data to be transferred betweendifferent switches, and ultimately between different communicationinterfaces coupled to different switches. Since the number ofcommunication interfaces that may be connected to a single switch isnormally limited, the switch interface, which enables data to betransferred from one switch to another allows switches, and thereforecommunication interfaces to be added as required.

[0006] In one embodiment, the switch interface means includes first andsecond communication ports for data communication, and each beingconnectable to a communication path. Advantageously, this enables theswitch to be connected into a communication path ring so that data maybe transferred between the switch and another switch via two possibleroutes.

[0007] In one embodiment, the switch interface means is adapted forenabling data to be transferred between the first and second ports.Advantageously, this arrangement provides continuity of thecommunication path in which the switch is connected so that data whichis not intended for the switch can be readily transferred across theswitch.

[0008] In one embodiment, the switch interface means includes one ormore further communication ports, and the switch interface means isconditioned to enable data to be transferred between any of thecommunication ports. Advantageously, this arrangement enables data to betransferred from one communication path to another communication path.

[0009] In one embodiment, the switch interface means includes a firstswitch interface having a first communication port for connection to acommunication path and a second switch interface having a secondcommunication port for connection to a communication path.

[0010] The first switch interface may be connected to the firstswitching unit and the second switch interface may be connected to thesecond switching unit. Advantageously, the provision of first and secondswitch interfaces each having an interface port connectable to the samecommunication path enables data to be transferred between the secondswitching unit and the local communication path independently of thefirst switch interface, so that if either of the first switching unit orthe first switch interface fails, data may still be transferred onto tothe local communication path and therefore to other similar switches.Thus, the switch provides a switch module having its own redundancycapability and to which other switch modules may be connected to providea robust, expandable and scaleable modular switching system.

[0011] In one embodiment, at least one of the first and second switchingunits is removable from the switch so that the other switching unit canoperate in its absence. This arrangement enables the switch to continueto operate in the absence of one switching unit so that a faultyswitching unit can be replaced without the switch causing a loss ofservice.

[0012] In one embodiment, the first and second switch interfaces areinterconnected to enable data to be transferred therebetween. In oneembodiment, the first and second switch interfaces may be adapted toenable data to be transferred between the first switching unit and thesecond switch interface. This arrangement may be adapted, for example,to enable data received by the first switching unit, and which isdestined for another switch coupled to the local communication path, tobe routed onto the first communication path either from the firstinterface port or the second interface port, thereby allowing the switchto route data to another switch along the most appropriate (e.g.shortest or most direct) section of the local communication path.

[0013] Similarly, the first and second interfaces may be adapted toenable data received by the second switching interface from the localcommunication path to be transferred to the first switching unit via thefirst switch interface.

[0014] In another embodiment, the first and second switch interfaces maybe adapted to enable data to be transferred between the second switchingunit and the first switch interface, so that, for example data receivedby the second switching unit destined for the local communication pathcan be routed onto the local communication path via either of the firstand second interface ports, so that the appropriate route for that datacan be selected. The first and second switch interface may be arrangedto enable data received by the first switch interface from the localcommunication path to be directed to the second switching unit via thesecond switch interface. The first switching unit may be detachable fromthe switch independently of the first switch interface so that data cancontinue to be transferred between the first and second switchinterfaces so that the first switch unit can be replaced or repairedwithout affecting the quality of service.

[0015] In another embodiment, the first and second switch interfaces maybe arranged to enable data to be transferred between the first andsecond interface ports. Advantageously, this arrangement enables datawhich is transported along the local communication path, but which isnot intended for the switch, to pass through the switch and return tothe local communication path.

[0016] According to another aspect of the present invention, there isprovided a switching apparatus for a communication network comprising aplurality of switch modules, each switch module comprising a switchingunit for connection to at least one communication interface, a switchinterface for communication with said switching unit and having firstand second communication ports, and wherein the first communication portof each switch module is connected to the second communication port ofanother switch module such that said switch modules are connected in afirst communication path ring.

[0017] According to another aspect of the present invention, there isprovided a switching circuit for a communication network switch,comprising a switching unit having a plurality of ports, each forconnection to a communication interface, the switching units beingadapted to direct data between the ports, a switch interface coupled tothe switching unit and having at least one communication port, theswitch interface being adapted to transfer data between the switchingunit and the at least one communication port.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Examples of embodiments of the present invention will now bedescribed with reference to the drawings, in which:

[0019]FIG. 1 shows a schematic diagram of a switch according to anembodiment of the present invention;

[0020]FIG. 2 shows a block diagram of an array of interconnected switchmodules according to an embodiment of the present invention;

[0021]FIG. 3 shows a block diagram of a switch module according to anembodiment of the present invention;

[0022]FIG. 4 shows a schematic diagram of interconnected switchinterfaces according to an embodiment of the present invention;

[0023]FIG. 5 shows a modular switch having a two dimensional array ofinterconnected switch modules according to an embodiment of the presentinvention;

[0024]FIG. 6 shows a schematic block diagram of a three-dimensionalarray of interconnected switch modules according to an embodiment of thepresent invention;

[0025]FIG. 7 shows a diagram of a switch module according to anotherembodiment of the present invention;

[0026]FIG. 8 shows a simplified design of the embodiment shown in FIG.7, and

[0027]FIG. 9 shows a diagram of an array of interconnected switchingmodules of FIGS. 7 and 8 according to another embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS

[0028] Referring to FIG. 1, a switch 1 for a communication network,according to an embodiment of the present invention, comprises a firstswitching unit 3, a first switch interface 5, a second switching unit 7and a second switch interface 9. The first and second switching units 3,5 each have a plurality of communication ports 11, 13, 15, 17, 19, eachfor connection to a communication interface 21, 23, 25, 27, 29. In thisembodiment, each communication interface is connected to a communicationport of the first switching unit 3 and a communication port of thesecond switching unit 7.

[0029] The first switching unit and first switch interface areinterconnected to enable data to be transferred therebetween, and thefirst switch interface includes a local communication port 31 forconnection to a local communication path 33. The second switching unit 7and the second switch interface 9 are also interconnected to enable datato be transferred therebetween, and the second switch interface 9 has alocal communication port 35 for connection to the local communicationpath 33.

[0030] In this embodiment, each of the first and second switching unitsare capable of providing local switching to enable data to betransferred between different communication interfaces 21, 23, 25, 27,29. Each switching unit 3, 7 is also capable of routing data receivedfrom the communication interfaces to its associated switch interface 5,9 for transmission along the local communication path 33, and is alsocapable of directing data to the appropriate communication interfacefrom the local communication path via its associated switch interface 5,9. In operation, one of the first and second switching units 3, 7 isactive to provide switching functions and the other is inactive but canbe invoked to provide the same switching functions as the active unit,if the active unit fails or experiences a fault. For example, the firstswitching unit 3 may serve as the active unit and the second switchingunit 7 may serve as the standby unit (or vice versa). In this case, thefirst switching unit 3 provides local switching to enable data to betransferred between different communication interfaces 21, 23, 25, 27,29 connected thereto, and to direct data which is intended for the localcommunication path 33 to the first switch interface 5.

[0031] In this embodiment, the first and second switch interfaces 5, 9are interconnected by a transfer interface 37 which enables datareceived by the first switch interface from the first switching unit 3to be transferred to the second switch interface 9, and data received bythe second switch interface 9 from the local communication path 33 to betransferred to the first switching unit 3 via the first switch interface5. This arrangement enables the switch to route data received by thefirst switching unit 3 along the local communication path either fromthe local communication port 31 of the first switch interface 5 or fromthe local communication port 35 of the second switch interface 9.

[0032] In this embodiment, the switch 1 further includes a passthroughinterface 39 interconnecting the first and second switch interfaces 5, 9and which enables data received by the first switch interface from thelocal communication path 33 to be transferred to the second switchinterface 9 and back onto the local communication path 33, and viceversa. In this way, data which is not intended for any of thecommunication interfaces to which the switch is connected can passthrough the switch to its intended destination.

[0033] In the event that the first switching unit 3 fails, experiences afault or is otherwise de-activated, the second switching unit 7 can beinvoked to provide the required switching functions. In this case, thesecond switching unit 7 can perform local switching functions to enabledata to be transferred between different communication interfaces 21,23, 25, 27, 29, and can pass data from the communication interfacesintended for the local communication path to the second switch interface9, and pass data received by the second switch interface 9 to theintended communication interface. The second switch interface 9 directsdata intended for the section of the local communication path to whichit is connected to its local communication port 35, and directs dataintended for the section of the local communication path to which thefirst switch interface is connected to the first switch interface 5 viathe transfer interface 37. In this way, the switch is capable ofcontinuing to perform all of its switching functions without loss ofservice and without a degradation in the quality of service if theactive switching unit fails.

[0034] In one embodiment, the first switching unit 3 can be removed fromthe switch independently of the second switching unit, so that theswitch can continue to perform its switching functions while the firstswitching unit is replaced. In one embodiment, the first switching unitmay be removable from the switch independently of the first switchinterface 5, so that the switch can continue to operate in the absenceof the first switching unit without a degradation in the quality ofservice.

[0035] In another embodiment, the first switch interface 5 may beseparable from the second switch interface 9 and removable from theswitch independently of the second switch interface. For example, thefirst and second switch interfaces may be mounted on separate orseparable substrates as indicated by the dashed line 40. Alternatively,they may be mounted on the same substrate and individually removabletherefrom. The first switch interface may further be removable from theswitch independently of the first switching unit or may only beremovable from the switch with the first switching unit. In either case,the switch can continue to operate without the first switching unit andthe first switch interface, allowing both the first switching unit andswitch interface to be removed from the switch and replaced, ifrequired. In this case, the second switching unit 7 is activated andprovides local switching functions between different communicationinterfaces and passes data intended for the local communication path tothe second switch interface 9. As the first switch interface 5 isdisconnected from the second switch interface 9, all data which isintended for the local communication path is transmitted onto the localcommunication path from the second switch interface 9. However, as otherswitches connected to the local communication path are accessiblethrough either of the first switch interface or the second switchinterface, the switch is still operative to transmit data to and receivedata from any other switch which is also connected in the localcommunication path so that switch-to-switch connectivity is retained.The loss of the first switch interface 5 means that data from the switchcan only be launched onto the local communication path in one directionwhich may not be the most efficient or effective route for transmittingdata to a particular switch, and may for example require the data topass over a longer section of the local communication path or through alarger number of switches than would otherwise be required if the datacould have been transferred onto the local communication path via thefirst switch interface, and this may or may not affect the quality ofservice, depending on the data traffic density and the bandwidth orcapacity of the local communication path.

[0036] Similarly, data intended for the switch transmitted by anotherswitch connected to the first communication path may be transmitted tothe switch along the route which is connected to the local communicationport 35 of the second switch interface. Again, since the option oftransmitting the data along the local communication path to the firstswitch interface, in the opposite direction, is not available, thequality of service may or may not be affected, again depending on thelength of the local communication path between the transmitting andreceiving switches, the number of intermediate switches between thetransmitting and receiving switches, the actual traffic density and thebandwidth or capacity of the local communication path. As mentionedabove, the switch may be interconnected with one or more similarswitches to form an enlarged modular switching system that may bereadily expanded or contracted simply by adding or removing one or moreswitch modules. An example of a modular switching system having aplurality of interconnected switch modules is shown in FIG. 2.

[0037] Referring to FIG. 2, a modular switching system 101 comprises aplurality of switch modules 103, 105, 107, 109. Each switch module,which may be similar to the switch described above and shown in FIG. 1,has a first switching unit 111, a first switch interface 113, a secondswitching unit 115 and a second switch interface 117. Each of the firstand second switching units 111, 115 are coupled to respective first andsecond switch interfaces 113, 117 to pass data therebetween, and eachswitch module has a transfer interface 112 and a passthrough interface114 for passing data between the first and second switch interfaces 113,117. As for the embodiment described above and shown in FIG. 1, thetransfer interface passes data derived from or intended for acommunication interface, and the passthrough interface 114 passes dataderived from or intended for one or more local communication path(s).The first and second switching units 111, 115 are connected to aplurality of communication interfaces 119, 121, 123, 125. Each of thefirst switch interfaces 113 has a local communication port 127, 129,131, 133 for connection to a local communication path, and each of thesecond switch interfaces 117 also have a local communication port 135,137, 139, 141 for connection to the local communication path.

[0038] Considering the modular switching system as initially having justtwo switch modules 103, 105, the local communication port 127 of thefirst switch interface 113 of the first module 103 is connected to thecommunication port 137 of the second switch interface 117 of the secondmodule 105 by a communication link 143, and the local communication port135 of the second switch interface 117 of the first module 103 isconnected to the communication port 129 of the first switch interface ofthe second module 105. Thus, data may be transferred between the firstand second switch modules 103, 105 via two possible routes, andtherefore the local communication path, which includes the first andsecond links 143, 145, and which interconnects the first and secondmodules 103, 105, has a ring or toroid-like topology. If one of theswitching units 111, 115 of either switch module 103, 105 fails, datamay continue to be transferred between the switch modules via either ofthe two possible routes. On the other hand, if one of the switchinterfaces 113, 117 fails or is removed, so that data can no longer betransmitted onto the communication link to which it is connected, datamay still be transferred between the switch modules using the otherswitch interface and its associated communication link.

[0039] Adding a second switch module 105 to the first switch moduleeffectively provides a switch having an increased number of availablecommunication ports and therefore increases the number of communicationinterfaces (e.g. line cards) that can be connected to the modularswitch. If the second module has the same number of communication portsas the first, adding a second module effectively doubles the number ofavailable communication ports.

[0040] In order to add a third switch module 107, the firstcommunication link 143 may simply be disconnected from the localcommunication port 137 of the second switch interface of the secondmodule 105, and connected to the local communication port 139 of thesecond switch interface 117 of the third module 107, and the localcommunication port 137 of the second switch interface 117 of the secondmodule 105 may be connected to the local communication port 131 of thefirst switch interface 113 of the third module 107 by a thirdcommunication link 147. The communication link 143 may be extended ifrequired, or may be replaced altogether with another link bydisconnecting the original link also from the local communication port127 of the first module 103.

[0041] In this way, the third switch module 107 is connected into thelocal communication path loop or ring, such that data can be passed fromthe third switch module 107 onto the ring in either direction, i.e.either onto communication link 147 connected to the second switch module105, or onto the first communication link 143 connected directly to thefirst switch module 103. Similarly, data can be received by the thirdswitch module from the local communication path from either directionalong the ring, i.e. over the third communication link 147 or over thefirst communication link 143.

[0042] This interconnect scheme also allows data to be transferredbetween one switch module and any other switch module in eitherdirection along the local communication path loop. For example, data maybe transferred from the third switch module 107 to the first switchmodule 103 either directly via the first communication link 143 or viathe other part of the loop, which includes the second module 105, inwhich case data is transferred from the first switch interface 113 ofthe third switch module 107 to the second switch interface 117 of thefirst switch module 103 via the third communication link 147, the secondswitch interface 117 of the second module, the passthrough interface114, the first switch interface 113 of the second module and the secondcommunication link 145. Thus, the first and second switch interfaces113, 117 and the passthrough interface 114 may effectively constitutepart of the local communication path.

[0043] It is to be noted that since the second communication link 145can remain intact while the modular switch is expanded to include thethird switch module 107, data can continue to be transferred between thefirst and second switch modules, without loss of service, although thefirst and/or second switch modules may need to be adapted such that anydata which is to be transferred between the first and second switchmodules is passed over the second communication link 145 rather than thefirst communication link 143.

[0044] Assuming the first switching unit 111 of the third switch module107 serves as the active switch unit, and the second switch unit 115serves as a standby switch unit, if the active switch unit 111 fails oris removed from the switch module, the standby switch unit 115 can beactivated, and communication can continue between the switch module andits associated communication cards 123. If the first switch interface113 of the third switch module 107 fails, or is removed from the switchmodule, the standby switch unit 115 can be activated, and datatransferred between the communication interfaces 123 and the othercommunication interfaces 119, 121 associated with the other modules overthe first communication link 143 of the local communication loop.

[0045] The modular switch 101 may be expanded further to include afourth switch module 109 (and subsequent switch modules, as required).The fourth switch module 109 may be connected into the localcommunication path loop by disconnecting the first communication link143 from the local communication port 139 of the second switch interface117 of the third switch module and connecting the first communicationlink 143 to the local communication port 141 of the second switchinterface 117 of the fourth switch module 109, and connecting the localcommunication port 139 of the second switch interface 117 of the thirdswitch module 107 to the local communication port 133 of the firstswitch interface 113 of the fourth switch module 109 by a fourthcommunication link 149.

[0046] Adding a fourth switch module only requires one of the twopossible communication paths between the first and third switch moduleto be temporarily broken, the other path, which is provided by thesecond and third communication links 145, 147 and the passthroughinterface between the first and second switch interfaces of the secondswitch module 105, remaining in situ. Therefore, while the fourth switchmodule is being added into the modular switch, data can continue to betransfer between any of the first, second and third switch modules,although the switch modules may need to be adapted to direct dataintended for another switch module over the section of the localcommunication loop which remains intact.

[0047] It can be seen that, once the fourth switch module 109 has beeninstalled, the fourth switch module has access to each of the first,second and third switch modules via two possible routes. For example,the fourth switch module 109 has access to the second switch module 105either via the first and second communication links 143, 145 and thepassthrough interface of the first switch module, or via the third andfourth communication links 147, 149 and the passthrough interface of thethird switch module 107.

[0048] Assuming the first switching unit 111 of the fourth switch module109 serves as the active switch unit and the second switch unit 115serves as the standby switch unit, if the active switch unit 111 failsor is removed from the switch module, the standby switch unit 115 can beinvoked and communication can continue between the switch module and itsassociated communication cards 125. If the first switch interface 113 ofthe fourth switch module 109 fails or is removed from the switch module,the standby switch unit 115 can be activated, and data transferredbetween the communication interfaces 125 and the other communicationinterfaces 119, 121, 123 associated with the other switch modules overthe first communication link 143 of the local communication loop. Thus,the switch module according to embodiments of the present inventionenable a modular switch to be built and expanded, as required, and thelocal redundancy capability of each module significantly enhances theresiliency and preserves the integrity of the modular switch in theevent that a switching unit and/or a switch interface of a module failsor is removed from the module. Advantageously, the system also allows aservice provider to expand and scale the capability of the communicationswitch incrementally and only as required, thereby obviating the needfor large initial investments in the purchase of a single large capacityswitch (or two large capacity switches for redundancy) whose maximumcapacity is fixed and therefore cannot be scaled and whose totalswitching capacity may be underused for some time.

[0049] In another embodiment of the present invention, the switch mayhave additional communication ports to enable the switch to beinterconnected to other switches via one or more other localcommunication paths. Advantageously, this increases the flexibility withwhich additional switches can be added and also enables the degree ofconnectivity between one switch and another switch to be increased byincreasing the number of possible communication paths by which theswitches are interconnected. Increasing the number of potentialcommunication paths connecting any two switches may both increase thetraffic carrying capacity of a modular switch formed by these switchmodules and also increase its resilience to a fault or failure in one ofthe modules. An example of a switch module having a plurality ofcommunication ports for connection to a plurality of different localcommunication paths is shown in FIG. 3.

[0050] Referring to FIG. 3, a switch module 201 comprises a firstswitching unit 203, a first switch interface 205, a second switchingunit 207 and a second switch interface 209. Each of the first and secondswitching units 203, 207 are coupled to a plurality of communicationinterfaces (e.g. line cards) 211, 213, 215, 217, 219, 221, 223 and 225.Each switching unit provides a local switching function and controls thetransfer of data between the various communication interfaces.

[0051] Each of the first and second switch interfaces 205, 209 has aplurality of local communication ports for enabling data to betransferred into and out of the switch 201 from and to a plurality ofdifferent local communication paths. In the present embodiment, thefirst and second switch interfaces each have four local communicationports 227, 229, 231, 233, 235, 237, 239, 241, allowing the switch 201 tobe connected directly to eight other switches 247 to 261.

[0052] The first and second switch interfaces 205, 209 areinterconnected by a transfer interface 243 and by a passthroughinterface 245.

[0053] In operation, one of the switching units 203, 207 serves as theactive unit and the other as the standby unit which may be activated totake over switching functions from the active unit if the active unitfails. Assuming for the purposes of illustration, that the firstswitching unit 203 is the active unit, the switching unit directs datawhich is intended for another switch to the first switch interface 205.The first switch interface 205 serves to direct data onto its localcommunication ports 227, 229, 231, 233, as appropriate, and to directdata received from the first switching unit 203 which is intended fortransmission onto the local communication paths connected to the secondswitch interface 209 to the second switch interface 209 via the transferinterface 243.

[0054] In one interconnect scheme, the switch may be connected in fourdifferent local communication path rings or loops. Thus for example, thefirst local communication ports 227, 235 of the first and second switchinterfaces may be connected in a first local loop or toroid W, thesecond local communication ports 229, 237 of the first and second switchinterfaces may be connected in a second local loop X, the third pair oflocal communication ports 231, 239 connected in a third local loop Y andthe fourth pair of local communication ports 233, 241 connected in afourth local loop Z. Thus, the switch is able to direct data onto anyone of the four local rings W, X, Y, Z in either direction, designated+W, +X, +Y, +Z if transferred to or from the first switch interface 205,and designated −W, −X, −Y and −Z, if transferred to or from the secondswitch interface 209.

[0055] The passthrough interface 245 enables data received by one switchinterface 205, 209 from the local communication loops or toroids to betransferred to the other switch interface and transmitted onto the localcommunication loops or toroids connected thereto.

[0056] In this embodiment, each switch interface 205, 209 is alsocapable of transferring data received from one communication path onto adifferent communication path. Thus, the first switch interface 205 iscapable of transmitting data received on any of the local communicationpaths +W, +X, +Y and +Z to which it is connected onto any othercommunication path +W, +X, +Y or +Z, and similarly, the second switchinterface 209 is capable of directing data received on any one of thefour communication paths −W, −X, −Y, −Z to which it is connected ontoany other communication −W, −X, −Y, −Z. The first and second switchinterfaces, together with the passthrough interface may be arranged toenable data received at any one of local communication ports of one ofthe switch interfaces 205, 209 to be directed on to any one of the localcommunication ports of the other switch interface. This ability toperform inter-loop transfers in either direction significantly increasesthe flexibility and number of possible routes over which data can betransmitted between different switch modules.

[0057]FIG. 4 shows an embodiment of the first and second switchinterfaces in more detail. The first switch interface 301 comprises aplurality of interconnected interface devices 303, 305, 307, 309. Eachinterface device has a local communication port 311, 313, 315, 317 forconnection to a local communication path. Each communication portenables bidirectional communication between the interface device and thecommunication path to which it is connected, and has an input 319 and anoutput 321. However, in other embodiments, one or more communicationports may be unidirectional. In this embodiment, the first switchinterface has four interface devices each of which is interconnected tothe other three interface devices to enable data to be transferred fromone interface device to any other interface device so that data receivedon one communication path (or dimension) can be transferred to anothercommunication path (or dimension).

[0058] Similarly, the second switch interface 323 has a plurality ofinterconnected interface devices 325, 327, 329, 331 each having a localcommunication port 333, 335, 337, 339 for connection to a localcommunication path. Each communication port enables bidirectionalcommunication between its respective interface device and thecommunication path to which it is connected and has an input 319 and anoutput 321. However, in other embodiments, one or more communicationports may be unidirectional. In this embodiment, the second switchinterface 323 has four interface devices each of which is interconnectedto the other three interface devices to enable data to be transferredfrom one interface device to any other interface device. This enablesdata to be transferred from one local communication path (or dimension)to any other local communication path (or dimension).

[0059] Each of the interface devices 303, 305, 307, 309 of the firstswitch interface 301 and each of the interface devices 325, 327, 329,331 of the second switch interface 323 are also interconnected via apassthrough interface 341 which enables data to be transferred betweenany interface device of the first switch interface 301 and any interfacedevice of the second switch interface 323.

[0060] Each of the interface devices of the first switch interface 301also has a data transfer port 343, 345, 347, 349 for data transferbetween the device and the first switch unit (for example, unit 203shown in FIG. 3), and each of the interface devices of the second switch323 also has a data transfer port 351, 353, 355, 357 for data transferbetween the interface device and the second switching unit (for exampleunit 207 shown in FIG. 3).

[0061] It is to be noted that a transfer interface for enabling data tobe transferred between a switching unit connected to a respective switchinterface and the other interface may also provided but is omitted fromFIG. 4, for clarity.

[0062] In one embodiment, each interface device includes a system formanaging the transfer of data received from the local communication pathto which it is connected or from the passthrough interface to anotherinterface device, and a system for managing the transfer of datareceived from another interface device onto the local communication pathto which it is connected. For example, each interface device may includea memory or buffer for temporarily storing data received from its localcommunication path and/or the passthrough interface prior to itstransmission to another interface device. Each interface device may alsoinclude a scheduler for managing the transfer of data from each of theother interface devices onto its local communication path. For example,the scheduler of one interface device may be coupled via a data transferbus to the memory or buffer of another interface device so that data canbe transferred from the memory under the control of the outputscheduler. A separate control bus may be provided between one interfacedevice and another for carrying control signals required to control datatransfer from the memory or buffer of one interface device to the outputscheduler of another interface device. Examples of interface systems forenabling data to be transferred from one interface device to another isdescribed in the applicant's copending Canadian Application No.2,366,397 filed on Dec. 31, 2001 (Attorney Docket No. 78945-41).

[0063] Although in the embodiment of FIG. 4, each switch interface hasfour local communication ports, other embodiments may have any othernumber of communication ports, for example one, two or more than two.The larger the number of local communication ports provided for a switchinterface, the higher the number of local communication paths to whichthe interface can be connected.

[0064] The provision of two local communication ports on each of thefirst and second switch interfaces enables each switch module to beconnected in two different local communication path loops or rings. FIG.5 shows a two-dimensional array of switching elements and illustrateshow the provision of two local communication ports on each of the firstand second switch interfaces of each switch module can be used in aninterconnect scheme which provides connectivity between every switchmodule in the array.

[0065] Referring to FIG. 5, a plurality of switch modules are arrangedin a two-dimensional array of three rows 404, 406, 408 and two columns410, 412. This arrangement may or may not be representative of how theswitch modules are actually arranged in an installation for example, ofracks or shelves. Each switch module may be similar to those describedpreviously, and include first and second switching units 414, 416, eachconnected to a plurality of communication interfaces 418 and first andsecond switch interfaces 415, 421. The first switch interface 415 ofeach switch module has a first and second local communication port 417,419, and the second switch interface 421 of each switch module has afirst and second local communication port 423, 425. The switchingmodules in each row are interconnected in a respective first localcommunication path loop 427, 429, 431 (each of which may be referred asan X-dimensional loop, shown by the solid lines) comprising a firstcommunication link 433, 435, 437 connecting the first localcommunication port 417 of the first switch interface 415 of each of thefirst switch modules 403, 407, 411 to the first local communication port423 of the second switch interface 421 of each of the second switchmodules 405, 409, 413 in each row, and a second communication link 439,441, 443 connecting the first local port 423 of the second switchinterface 421 of each of the first switch modules 403, 407, 411 of eachrow to the first local communication port 417 of the first switchinterface 415 of each of the second switch modules 405, 409, 413 in eachrow. Thus, the switch modules in each row are interconnected via twopossible communication paths provided by the loop or ring topology andare configured to enable data to continue to be transferred between theswitch modules in each row if one of the switching units and/or itsassociated switch interface fails or is removed from the module.

[0066] The second local communication ports of the switch modules 403,407, 411 in the first column 410 are used to interconnect these modulesin a second local communication path loop or ring 445, and which may bereferred to as a first Y-dimensional loop, shown by dashed lines. Thesecond local communication ports of the switch modules 405, 409, 413 inthe second column 412 are also used to interconnect these switch modulesin another local communication path loop or ring 447 which may bereferred to as a second Y-dimensional loop, also shown by dashed lines.The first Y-dimensional local communication path 445 includes a firstcommunication link 449 connected between the second local communicationport 425 of the second switch interface 421 of the first switch module403 in the first row and the second local communication port 419 of thefirst switch interface 415 of the first switch module 411 in the thirdrow, a second communication link 451 connected between the second localcommunication port 419 of the first switch interface 415 of the firstswitch module 403 in the first row and the second local communicationport 425 of the second switch interface 421 of the first switch module407 in the second row, and a third communication link 453 connectedbetween the second local communication port 419 of the first switchinterface 415 of the first switch module 407 in the second row and thesecond local communication port 425 of the second switch interface 421of the first switch module 411 in the third row. This communication path445 (which also includes the passthrough interface between the first andsecond switch interfaces of each switch module) interconnects eachswitch module 403, 407, 411 in the first column to the other switchmodules in the first column via two possible routes so that data cancontinue to be transferred between each switch module in the event thata switching unit and/or switch interface of one of the modules fails oris removed from the module.

[0067] Similarly, the local communication path 447 interconnecting theswitch modules 405, 409, 413 in the second column 412 comprises a firstcommunication link 455 interconnecting the second local communicationports of the second switch interface 421 of the second switch module 405in the first row and the first switch interface 415 of the second switchmodule 413 in the third row, a second communication link 457interconnecting the second communication ports of the first switchinterface 415 of the second switch module 405 in the first row and thesecond switch interface 421 of the second switch module 409 in thesecond row, and a third communication link 459 interconnecting thesecond local communication ports 419, 425 of the first switch interface415 of the second switch module 409 in the second row and the secondswitch interface 421 of the second switch module 413 in the third row.Again, this interconnect scheme (together with the passthroughinterfaces) ensures that each of the switch modules in the second columnare interconnected to the other switch modules in the second column viatwo possible routes so that if one of the switching units and/or itsassociated switch interface of one of the switch modules fails or isremoved, data can continue to be transferred between the modules.

[0068] In this embodiment, each switch interface is capable oftransferring data between its respective first and second localcommunication ports, which allows data to be transferred betweendifferent local communication loops, and therefore between switchmodules positioned in both different rows and different columns. Forexample, data may be transferred from the first switch module 403 in thefirst row to the second switch module 409 in the second row bytransferring data over the first communication path 427 to the secondswitch module 405 of the first row, using either of the two possibleroutes and from the second switch module 405 onto the localcommunication path 447 to the second switch module 409 in the secondrow, again using either of the two possible communication path routes.Alternatively, data may be transferred from the first switch module 403of the first row to the second switch module 409 of the second row viaany one of the first switch module 407 of the second row, or via thefirst and second switch modules 411, 413 of the third row.

[0069] As can be appreciated from this example, each of the switchmodules are interconnected to each of the other switch modules in thearray via a number of different diverse routes. The degree ofconnectivity between the switch modules in the array may be increased byincreasing the number of local communication ports provided for eachswitch interface. For example, if a third local communication port isprovided for each switch interface, this can be used to interconnectswitch modules contained in both different rows and different columns.

[0070] In another embodiment, a switch module may have at least threelocal communication ports for each switch interface, and each port maybe used to connect the module to other switch modules in athree-dimensional array, with each communication port connecting theswitch module to another module along a local communication path in eachof three dimensions. An example of a modular switch having a pluralityof switch modules arranged in a three-dimensional array as shown in FIG.6. The modular switch 501 has a plurality of switch modules 503 havingfirst and second switch interfaces,(not shown) each having at leastthree communication ports for enabling the switch module 503 to beconnected in three local communication path loops. For example, a firstswitch module 505 is connected in a first local communication path 507which extends along the X-dimension, and which may include any number offurther switch modules, and, in this embodiment, connects the firstswitch module 505 to a second switch module 509 on the X-dimensionalpath. The first switch module 505 is connected in a second localcommunication path 511 which extends along the Y-dimension, and connectsthe first switch module to a neighbouring switch module 513. The firstswitch module 505 is also connected in a third local communication path515 which extends along the Z-dimension and connects the first switchmodule 505 to one or more other switch modules 517, 519 which are alsoconnected in the Z-dimensional communication path.

[0071] Preferably, each of the first and second switch interfaces ofeach module has the capability of transferring data between each of thecommunication ports so that the interfaces can transfer data between thedifferent local communication paths to which it is connected. Thus, eachswitch module can direct data traffic along any one of three dimensionsin either direction, and this enables data to be transferred from oneswitch module to any other switch module within the three-dimensionalarray over a number of different possible routes.

[0072] The local communication paths in which switching modules areconnected may either have a loop or ring topology, therebyinterconnecting each switch module to another switch module via twopossible routes as exemplified by local communication path loops 523,525, 527 shown in FIG. 6, or switch modules may be interconnected by asingle path, as exemplified by communication paths 529, 531, in FIG. 6,although a second path may be added to these paths to provide a loop orring topology. Thus, a modular switch may include local paths all ofwhich have a ring topology, or all of which have a single interconnecttopology, or a mixture of these.

[0073] In another embodiment, the switch module may include one or morefurther sets of first and second switch interfaces to increase thecapacity of the switch module, and for example increase the number ofcommunication interfaces that can be connected to the switch module. Anexample of a switch module having a second set of first and secondswitch interfaces is shown in FIG. 7. Referring to FIG. 7, a switchmodule 601 comprises a first switching unit 603 and a second switchingunit 605, each connected to a plurality of communication interfaces 607,609, 611, 613, 615, 617, 619, 621. The switch module 601 includes afirst set 623 of first and second switch interfaces 625, 627 and asecond set 629 of first and second switch interfaces 631, 633. In thisembodiment, the first and second set of switch interfaces 623, 629 sharethe data transfer requirements of the switching units, 603, 605. Thefirst switch interface 625 of the first set 623 is coupled to the firstswitching unit 603 and handles the data transfer requirements of thefirst four communication interfaces 607, 609, 611, 613. The first switchinterface 631 of the second set 629 is also coupled to the firstswitching unit 603 and is arranged to handle the data transferrequirements of the second set of four communication interfaces 615,617, 619, 621. Similarly, the second switch interface 627 of the firstset 623 is connected to the second switching unit 605, and is arrangedto handle the data transfer requirements of the first set of fourcommunication interfaces 607, 609, 611, 613. The second switch interface633 of the second set 629 is also coupled to the second switching unit605 and is arranged to handle the data transfer requirements of thesecond set of four communication interfaces 615, 617, 619, 621. In thisembodiment, each switch interface has four local communication ports635, 637, 639, 641, which enable each interface to be connected to fourlocal communication paths W, X, Y, Z.

[0074] In one implementation, respective pairs of first, second, thirdand fourth communication ports 635, 637, 639, 641 of the first switchinterfaces 625, 631 of the first and second sets may be connected to thesame side of a respective communication path, indicated by +W, +X, +Yand +Z. For example, the first local communication port 635 of each ofthe first switch interfaces 625, 631 may be coupled to the same side ofthe communication path W, indicated as +W in FIG. 7, and the secondlocal communication ports 637 of the first switch interfaces 625, 631are coupled to the same side of the local communication path X,indicated as +X, as shown in FIG. 7. Similarly, respective pairs offirst, second, third and fourth local communication ports 635, 637, 639,641 of the second switch interfaces 627, 633 may also be coupled to thesame side of a respective communication path, and which is the otherside of the communication path to which each of the local communicationports of the first switch interfaces are connected, and are designated−W, −X, −Y and −Z. In this embodiment, one of the local communicationports 635 of each switch interface is used to interconnect theinterfaces of the first set 623 to the interfaces of the second set 629.Thus, the local communication port 635 of the first switch interface 625of the first set is connected to the local communication port 635 of thesecond switch interface 633 of the second set 629 by a communicationlink 643, thereby forming a first communication path W, and the localcommunication port 635 of the second switch interface 627 of the firstset 623 is connected to the local communication port 635 of the firstswitch interface 631 by a communication link 645, thereby forming asecond communication path W.

[0075] A simplified diagram of the embodiment of FIG. 7 is shown in FIG.8, and like parts are designated by the same reference numerals. A firstpassthrough interface 647 is provided to enable data to be transferredbetween the first and second switch interfaces 625, 627 of the firstset, and a second passthrough interface 649 is provided to enable datato be transferred between the first and second switch interfaces of thesecond set. As shown in FIG. 7, the first switch interface 625 of thefirst set of switch interfaces is connected to the second switchinterface 633 of the second set via a communication link 643, and thesecond switch interface 631 of the first set is connected to the firstswitch interface of the second set 627 via a communication link 645.Advantageously, these interconnections provide direct connectivitybetween the first switch interface of one set with the second switchinterface of another set so that, together with the passthroughinterfaces 647, 649, data can be transferred from one switch interfaceof one set and any of the switch interfaces of another set withoutneeding to pass the data through either of the first or second switchingunits (or switching fabrics) 603, 605. For example, in the absence ofthe communication link 643, in order to pass data from the first switchinterface 625 of the first set to the second switch interface 633 of thesecond set, one possible route would be to pass data from the firstswitch interface 625 of the first set to the first switch interface 631of the second set through the first switching unit 603, and then to passthe data from the first switch interface to the second switch interfaceof the second set via the passthrough interface 649. However, theprovision of a direct communication link 643 between the first switchinterface 625 of the first set and the second switch interface 633 ofthe second set allows both the first switching unit 603 and the secondpassthrough interface 649 to be by-passed. Each of the othercommunication paths X, Y and Z connected between the first and secondswitch interfaces of a switch module may be implemented as two separatephysical paths when the switch module contains two sets of first andsecond interfaces. For example, as shown in FIG. 8, the switch module601 is connected in a Y dimensional communication loop, comprising afirst communication link 651 connected between the first and secondswitch interfaces 625, 627 of the first set, and a second communicationlink 653 connected between the first and second switch interfaces 631,633 of the second set. Again, the communication links 643, 645, whichform a W-dimensional communication path allows data to be transferredbetween the first and second Y-dimensional paths 651, 653.

[0076]FIG. 9 shows a one-dimensional array of interconnected switchmodules having first and second sets of switch interfaces. The onedimensional array 661 comprises first, second and third switch modules663, 665, 667, each of which is similar to the switching module shown inFIG. 8, and like parts are designated by the same reference numerals.Each switch module has two sets of first and second switch interfaces,the first switch interface of each set being connected to the secondswitch interface of the other set via a Y-dimensional communication path669 comprising communication links 643, 645, as for the embodiment shownin FIGS. 7 and 8. The switch modules are connected in a communicationpath loop or ring 671 shown by the dashed lines and designated as aY-dimensional path. The communication path loop 671 comprises first andsecond physical paths 673, 675, one of which interconnects first sets offirst and second switch interfaces of the switch modules and the otherof which interconnects the second sets of first and second switchinterfaces of the switch modules. Thus, the first path 673 includes afirst communication link 677 which connects the second switch interface627 of the first set of the first switch module 633 to the first switchinterface 625 of the first set of the third switch module 667, a secondcommunication link 679 connected between the second switch interface 637of the first set of the third switch module 667 and the first switchinterface 625 of the first set of the second switch modules 665, and athird communication link 681 connected between the second switchinterface 627 of the first set of the second switch modules 665 and thefirst switch interface of the first set of the first switch module 663.

[0077] The second path 675 comprises a first communication link 683connected between the second switch interface 633 of the second set ofthe first switch module 663 and the first switch interface 631 of thesecond set of the third switch module 667, a second communication link685 connected between the second switch interface 633 of the second setof the third switch module 667 and the first switch interface 631 of thesecond set of the second switch module 665, and a third communicationlink 687 connected between the second switch interface of the second setof the second switch module 665 and the first switch interface 631 ofthe second set of the first switch module 663.

[0078] The or each switching unit in any one of the embodimentsdescribed above may comprise any suitable circuit, device or fabric toperform the required function and may comprise a single device or aplurality of devices.

[0079] Although the switch interface means may comprise two separateswitch interfaces to enable one to be separated from the other, inanother embodiment the switch interface may comprise a unitary device,or a plurality of devices that are not readily separable from eachother.

[0080] The switch may be adapted to pass data cells or data packets toand from a communication network which may for example comprise anoptical or wireline network.

[0081] The switch and any components thereof may be adapted to directdata on to the appropriate route through the switch according to labelsassociated with the data.

[0082] The switch may be adapted for either unidirectional orbi-directional communication or a combination of both, either with thecommunication network or with any of the local communication paths.

[0083] The passthrough interface in any of the embodiments describedabove may be implemented such that data arriving at one of thecommunication ports of one of the switch interfaces is transferred fromthe interface device connected to that communication port to theinterface device of the other switch interface to which thecorresponding communication port is connected, and that interface devicemay be adapted to transfer the data to another interface device of thatother switch interface having a communication port to which the data isintended to be transferred.

[0084] Modifications and changes to the embodiments described hereinwill be apparent to those skilled in the art.

1. A switch for a communication network, comprising: a first switchingunit for communicating with one or more communication interfaces, asecond switching unit for communicating with said one or morecommunication interfaces, and switch interface means for communicatingwith said first and second switching units and having a communicationport for enabling at least one of data to be transferred to said switchinterface means and data to be transferred from said interface means. 2.A switch as claimed in claim 1, wherein at least one of said first andsecond switching units is capable of transferring data to said switchinterface means if the other of said first and second switching unitsexperiences a fault or failure.
 3. A switch as claimed in claim 1,wherein each of said first and second switching units has one or morecommunication ports for communicating with said one or morecommunication interfaces, and at least one of said first and secondswitching units is capable of transferring data received from saidswitch interface means to its one or more communication ports if theother of said first and second switching units experiences a fault orfailure.
 4. A switch as claimed in claim 1, wherein said switchinterface means includes first and second communication ports for datacommunication and each being connectable to a communication path.
 5. Aswitch as claimed in claim 4, wherein said switch interface means isadapted for enabling data to be transferred between said firstcommunication port and said second communication port.
 6. A switch asclaimed in claim 4, further including a local communication pathinterconnecting said first and second communication ports.
 7. A switchas claimed in claim 4, wherein said switch interface means includes oneor more further communication ports, and said switch interface means isconditioned to enable data to be transferred from any one of saidcommunication ports to any other of said communication ports.
 8. Aswitch as claimed in claim 4, wherein said switch interface meansfurther comprises third and fourth communication ports for connection toa communication path, and wherein said switch interface means isconditioned to enable at least one of data to be transferred betweensaid first communication port and said third communication port, anddata to be transferred between said second communication port and saidfourth communication port.
 9. A switch as claimed in claim 8, furtherincluding a first communication path interconnecting said first andsecond communication ports, and a second communication pathinterconnecting said third and fourth communication ports.
 10. A switchas claimed in claim 8, wherein said switch interface means furtherincludes fifth and sixth communication ports, and said switch interfacemeans is conditioned to enable data to be transferred between any ofsaid first, third and fifth communication ports, and to enable data tobe transferred between any of said second, fourth and sixthcommunication ports.
 11. A switch as claimed in claim 10, furtherincluding a first communication path interconnecting said first andsecond communication ports, a second communication path interconnectingsaid third and fourth communication ports, and a third communicationpath interconnecting said fifth and sixth communication ports.
 12. Aswitch as claimed in claim 8, wherein said switch interface means isadapted to enable data to be transferred between said third and fourthcommunication ports.
 13. A switch as claimed in claim 10, wherein saidswitch interface means is adapted to enable data to be transferredbetween said fifth and sixth communication ports.
 14. A switch asclaimed in claim 1, wherein said switch interface means includes aplurality of communication ports, and said switch interface means isadapted to enable data to be transferred from said first switching unitto any one of said communication ports.
 15. A switch as claimed in claim14, wherein said switch interface means is adapted to enable data to betransferred from said second switching unit to any one of said pluralityof communication ports.
 16. A switch as claimed in claim 1, wherein saidswitch interface means includes a first switch interface having a firstcommunication port for connection to a communication path and a secondswitch interface having a second communication port for connection to acommunication path and wherein said first and second switch interfacesare removable from said switch independently of the other.
 17. A switchas claimed in claim 16, wherein said first switch interface is coupledto said first switching unit to enable data to be transferredtherebetween, and said second switch interface is coupled to said secondswitching unit to enable data to be transferred therebetween.
 18. Aswitch as claimed in claim 16, further comprising coupling means forenabling data to be transferred between said first and second switchinterfaces.
 19. A switch as claimed in claim 18, wherein said couplingmeans is arranged to enable data to be transferred between said firstswitching unit and said second switch interface.
 20. A switch as claimedin claim 18, wherein said coupling means is arranged to enable data tobe transferred between said second switching unit and said first switchinterface.
 21. A switch as claimed in claim 16, further comprisingcoupling means for coupling said first switching unit to said secondswitch interface.
 22. A switch as claimed in claim 16, furthercomprising coupling means for enabling data to be transferred betweensecond switching unit and said first switch interface.
 23. A switch asclaimed in claim 16, further comprising coupling means for enabling datato be transferred between said first and second communication portsthrough said first and second switch interfaces.
 24. A switch as claimedin claim 16, wherein said first switch interface comprises one or morefurther communication ports, and said first switch interface is arrangedto enable data to be transferred between said communication ports.
 25. Aswitch as claimed in claim 24, wherein said second switch interfacefurther comprises one or more further communication ports, and saidsecond switch interface is arranged to enable data to be transferredbetween said communication ports.
 26. A switch as claimed in claim 25,further comprising coupling means for enabling data to be transferredbetween any one of said communication ports of said first switchinterface and any one of said communication ports of said second switchinterface.
 27. A switch as claimed in claim 16, wherein said firstswitch interface further includes a third communication port and saidsecond switch interface further includes a fourth communication port,and further including a communication path interconnecting said thirdcommunication port and said fourth communication port.
 28. A switch asclaimed in claim 16, wherein said second switch interface is adapted todirect data intended for said first communication port to said secondcommunication port in response to a fault or failure of said firstswitch interface or if said first switch interface is disconnected fromsaid second switch interface.
 29. A switch as claimed in claim 16,wherein said first switch interface is capable of transferring data fromsaid first switching unit intended for said first communication port tosaid first communication port and transferring data from said firstswitching unit intended for said second communication port to saidsecond switch interface.
 30. A switch as claimed in claim 16, whereinsaid second switch interface is capable of transferring data from saidsecond switching unit intended for said second communication port tosaid second communication port and transferring data from said secondswitching unit intended for said first communication port to said firstswitching unit.
 31. A switch as claimed in claim 1, wherein said firstand second switching units are each removable from said switchindependently of the other.
 32. A switch as claimed in claim 16, whereinsaid first switching unit is removable from said switch independently ofsaid first switch interface, such that said first switch interface cancontinue to operate.
 33. A switch as claimed in claim 16, wherein saidsecond switching unit is removable from said switch independently ofsaid second switch interface, such that said second switch interface cancontinue to operate.
 34. A switching apparatus for a communicationnetwork comprising a plurality of switch modules, each switch modulecomprising a switching unit for connection to at least one communicationinterface, a switch interface for communication with said switching unitand having first and second communication ports, and wherein the firstcommunication port of each switch module is connected to the secondcommunication port of another switch module such that said switchmodules are connected in a first communication path ring.
 35. Aswitching apparatus as claimed in claim 34, wherein at least one of saidswitch modules further includes a second switching unit forcommunication with said at least one communication interface, and forcommunication with the switch interface of the respective switch module.36. A switching apparatus as claimed in claim 35, wherein the or eachsecond switching unit is adapted to transfer data from said at least onecommunication interface to a respective switch interface if said firstswitching unit becomes inactive.
 37. A switching apparatus as claimed inclaim 35, wherein the or each second switching unit is adapted to passdata from a respective switch interface to said at least onecommunication interface if said first switching unit becomes inactive.38. A switching apparatus as claimed in claim 34, wherein the switchinterface of at least one of said switch modules is adapted to enabledata to pass between its respective first and second communicationports.
 39. A switching apparatus as claimed in claim 34, wherein saidplurality of switch modules includes a first switch module, the switchinterface means of said first switch module having a further pair ofcommunication ports, said apparatus further comprising a secondplurality of switch modules including said first switch module, andwherein one communication port of each pair of each switch module ofsaid second plurality is connected to the other communication port ofsaid pair of another switch module of said second plurality, such thatsaid second plurality of switch modules are connected in a secondcommunication path ring.
 40. A switching apparatus as claimed in claim39, wherein the switch interface means of at least one of said secondplurality of switch modules is adapted to enable data to be transferredbetween its respective pair of communication ports.
 41. A switchingapparatus as claimed in claim 39, wherein said first switch modulefurther includes an additional pair of communication ports for enablingsaid first switch module to be connected in a third communication pathring, said apparatus further comprising a third plurality of switchmodules, including said first switch module, and each having a pair ofcommunication ports, and wherein one of said pair of communication portsof each switch module of said third plurality is connected to the otherof said pair of communication ports of another switch module of saidthird plurality such that said third plurality of switch modules areconnected in a third communication path ring.
 42. A switching apparatusas claimed in claim 41, wherein the switch interface of at least one ofsaid switch modules of said third plurality is adapted to enable data tobe transferred between its respective pair of communication ports.
 43. Aswitching circuit for a communication network switch, comprising aswitching unit having a plurality of ports, each for connection to acommunication interface, said switching unit being adapted to directdata between said ports, a switch interface coupled to said switchingunit and having at least one communication port, said switch interfacebeing adapted to transfer data between said switching unit and said atleast one communication port.
 44. A switching circuit as claimed inclaim 43, wherein said switch interface includes a plurality ofcommunication ports, and is adapted to enable data to be transferredbetween each of said plurality of communication ports.
 45. A switchingcircuit as claimed in claim 43, wherein said switch interface furtherincludes a data transfer port and said switch interface is capable oftransferring data between said switching unit and said data transferport.
 46. A switching circuit as claimed in claim 43, wherein saidinterface further includes a data transfer port, and wherein said switchinterface is capable of transferring data between said at least onecommunication port and said data transfer port.
 47. A switching circuitas claimed in claim 43, further including a data transfer port forenabling data to be transferred between said switch interface and saiddata transfer port, and wherein said data transfer port includescoupling means for enabling said data transfer port to be coupled to adata transfer port of another said switching circuit.
 48. A switch for acommunication network, comprising first switching means forcommunicating with one or more communication interfaces, a first switchinterface for communicating with said first switching means and havingat least one interface port for connection to a local communicationpath, second switching means for communicating with said one or morecommunication interfaces, and a second switch interface forcommunicating with said second switching means and having at least oneinterface port for connection to said local communication path.
 49. Aswitch as claimed in claim 48, further including a local communicationpath interconnecting said interface ports of said first and secondswitch interfaces.
 50. A switch as claimed in claim 48, wherein at leastone of said first and second switching means is removable from saidswitch such that the other of said first and second switching means canoperate in the absence of the one switching means.
 51. A switch asclaimed in claim 48, wherein said first and second switch interfaces areinterconnected to enable data to be transferred therebetween.
 52. Aswitch as claimed in claim 51, wherein at least one of said first andsecond switch interfaces is removable from said switch such that theother of said first and second switch interfaces can operate to transferdata between a respective one of said first and second switch means anda respective interface port in the absence of the other of said firstand second switch interfaces.
 53. A switch as claimed in claim 51,wherein at least one of said first and second switch means can beremoved from said switch such that said first and second switchinterfaces can operate to transfer data therebetween.
 54. A switch asclaimed in claim 51, wherein said first and second switch interfaces areoperable to transfer data between their respective interface ports. 55.A switch as claimed in claim 51, wherein at least one of said first andsecond switch interfaces is adapted to transfer data from a respectiveone of said first and second switching means to the other of said firstand second switch interfaces.
 56. A switching apparatus for acommunication network comprising a plurality of switch modules, eachswitch module comprising first and second switching units for connectionto at least one communication interface, a switch interface forcommunication with each of said first and second switching units, andhaving a communication port, wherein the communication port of each ofsaid switch modules is connected to the communication port of anotherswitch module.